1. Field of the Invention
This invention relates to an electric motor which is particularly useful, for instance, as a fuel pump driving motor installed in a fuel tank of a motor vehicle.
2. Description of Related Art
Generally, a fuel pump is installed in the fuel tank of a motor vehicle and is driven with an electric motor to transport fuel. For example, there is a fuel-pump disclosed in U.S. Pat. No. 5,013,221 which comprises a pump chamber and an electric motor that controls the operation of the pump chamber. The motor rotates with electricity supplied to a planar commutator installed perpendicular to the rotation axis via brushes that are resiliently pressed on the commutator by a resilient member. Since a fuel pump is immersed in a fuel (for example, hydrocarbons) during operation, the pump must be corrosion-resistant to the fuel. For this purpose, the motors disclosed in U.S. Pat. No. 5,157,299 and Japanese Patent Laid-Open Publication No. Hei. 9-182381 employ a sintered carbon material for the contact members that is slidably in contact with the brushes of the commutator in order to provide corrosion resistance and to improve the durability of the commutator and brushes.
In such prior art electric motors, a brush holder that holds the brushes slidably on the contact face is opposed to the commutator in the axial direction. Thus, if the armature is pushed (due to shock) toward the brush holder, the carbon contact members of the commutator collide with the brush holder which may damage the carbon contact members.
It is, therefore, an object of the present invention to provide an electric motor having a structure for preventing a carbon contact member of a commutator from colliding with a brush holder and from being damaged even when an armature has moved in the axial direction toward the brush holder.
To attain this goal, the invention adopts the following techniques. The first aspect of the present invention provides an electric motor in which a limiter structure is formed to limit the movement of the armature toward the brush holder in the axial direction. The limiter structure is formed in at least either the brush holder or the armature to provide clearance between the contact member and the brush holder when the armature has been brought in contact with the limiter structure. Then, since there is no concern that the contact member of the commutator may collide with the brush holder even when the armature is jarred by shock toward the brush holder, it becomes possible to prevent the carbon contact member of the commutator from being damaged.
The electric motor may have a projection serving as a limiter structure that is formed on the brush holder and that is opposed to the armature. Then, since this limiter structure prevents the contact member of the commutator from colliding with the brush holder even when the armature is forced toward the brush holder, it becomes possible to protect the carbon contact member of the commutator from damage. In this case, the limiter structure can be easily obtained by forming a projection on the brush holder in a position opposed to an outer periphery of a resin mold of the armature or to a supporting member of the commutator. The electric motor may have a limiter structure that is a projection of either a whole or partial outer periphery of the resin mold of the armature. In this manner, the limiter structure can be easily formed.
Another aspect of the present invention provides an electric motor in which a limiter structure limiting the movement of an armature toward a brush holder in the axial direction is formed in at least either of the brush holder or the armature to meet a condition such that L1 less than L2. L1 is a distance between the limiter structure and an opposing contact face, and L2 is a distance between the contact member of the commutator and an opposing face of the brush holder. Then, since there is no concern that the contact member of the commutator may collide with the brush holder even when the armature is jarred by shock toward the brush holder, it becomes possible to prevent the carbon contact member of the commutator from being damaged.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.